Naturally occurring CD4+CD25+ regulatory T cells are important in controlling autoimmunity. Although the thymus appears to be a major site of CD4+CD25+ T cell development, it is still unclear whether there are also peripheral sites of development. We have found recently that CD4+CD25~ T cells can be converted into CD4+CD25+ regulatory T cells in vivo in the periphery. Conversion of CD4+CD25- T cells into CD4+CD25+ regulatory T cells may be a very important mechanism for peripheral CD4+CD25+ regulatory T cell homeostasis and maintenance, and a defect in this process could contribute to autoimmune disease development. We have found that prediabetic NOD mice have a normal percentage of CD4+CD25+ T cells in the thymus, but a significantly lower percentage in the periphery. Furthermore, NOD CD4+CD25- T cells are unable to convert in vivo into CD4+CD25+ cells that have regulatory function, strongly suggesting that NOD mice may have a defect in this conversion process and may, therefore, be unable to maintain this regulatory population. The central hypothesis of this proposal is that enhanced conversion of CD4+CD25- T cells into CD4+CD25+ regulatory T cells in NOD mice can prevent diabetes. We will test this hypothesis by studying the mechanisms and defects in conversion in NOD mice, determine whether this process can be restored and whether restoration is associated with amelioration of disease. By understanding the defects in conversion of CD4+CD25- T cells into CD4+CD25+ regulatory T cells in NOD mice, it may be possible to devise strategies to overcome these defects and, thereby, develop therapies to prevent or treat autoimmune disease.